Add more verification

[britter]

The plugin should verify that

• group and parameter names are all valid java identifiers (starting with a letter, only letters, numbers and underscores).
• group and parameter names are not reserved keywords, e.g. primitive type names, modifier names, "class"/"interface"/"enum"
• the name passed to fromEnvironment represents a valid environment variable name.
• the default value of an enum parameter is part of the values list